P(II) signal transduction proteins, pivotal players in microbial nitrogen control

Abstract

The P(II) family of signal transduction proteins are among the most widely distributed signal proteins in the bacterial world. First identified in 1969 as a component of the glutamine synthetase regulatory apparatus, P(II) proteins have since been recognized as playing a pivotal role in control of prokaryotic nitrogen metabolism. More recently, members of the family have been found in higher plants, where they also potentially play a role in nitrogen control. The P(II) proteins can function in the regulation of both gene transcription, by modulating the activity of regulatory proteins, and the catalytic activity of enzymes involved in nitrogen metabolism. There is also emerging evidence that they may regulate the activity of proteins required for transport of nitrogen compounds into the cell. In this review we discuss the history of the P(II) proteins, their structures and biochemistry, and their distribution and functions in prokaryotes. We survey data emerging from bacterial genome sequences and consider other likely or potential targets for control by P(II) proteins.

FIG. 1

Nitrogen regulation (Ntr) system of enteric bacteria. The activities of both GS and NtrC are regulated in response to the intracellular nitrogen status. UTase (glnD product) catalyzes the uridylylation and deuridylylation of P_II (glnB product). ATase catalyzes the adenylylation and deadenylylation of GS. NtrB catalyzes the phosphorylation and dephosphorylation of NtrC.

FIG. 2

Structure of the E. coli GlnB trimer viewed (top) from above and (bottom) from the side. Individual monomers are colored red, green, and blue.

FIG. 3

Comparison of the structures of E. coli GlnB and GlnK. Superimposition of the C^α traces for GlnB (black) and GlnK (red) with the position of the Tyr-51 residues in each molecule is indicated. Adapted from reference .

FIG. 4

Schematic representation of the potential mutiplicity of roles for P_II proteins. This cartoon summarizes information (both definitive and predicted) obtained from many different organisms.